Behind the barrier islands of the eastern and southern United States runs the Intracoastal Waterway (ICW), a 4,800 km (3,000 mi) chain of sounds, lagoons, tidal rivers and dug canals that lets boats travel from Annisquam, Massachusetts, around the tip of Florida to Brownsville, Texas, mostly without facing the open sea. It has two main parts, the Atlantic Intracoastal Waterway, which runs from Norfolk, Virginia, to Key West, and the Gulf Intracoastal Waterway, about 2,100 km (1,300 mi) from St. Marks, Florida, to the Mexican border. The U.S. Army Corps of Engineers maintains it, and the route is toll-free. Barges use it to carry petroleum, chemicals, building materials and food, and in 2018 the Gulf section alone moved 112 million tons, making it the third-busiest waterway in the country.
A Coast Lined With Barrier Islands and Lagoons
The ICW exists because of the shape of the coast. Much of the Atlantic and Gulf shoreline is fronted by barrier islands, long ridges of sand built by waves and tides parallel to the mainland. Behind them lie shallow sounds, lagoons and salt marshes, sheltered from ocean swell and linked to the sea only through gaps called inlets. Of the roughly 1,500 barrier islands identified worldwide, more than a third are in North America, and some of the largest examples are along the ICW's route. The Outer Banks of North Carolina run for about 320 km, and Padre Island in Texas is the longest barrier island in the world at 182 km.
Seen from the sea, this is a dangerous coast: hundreds of shipwrecks off the Outer Banks have earned those waters the nickname Graveyard of the Atlantic. Behind the islands, though, there was already a natural inside route, broken only where a neck of land or a stretch of marsh separated one sound from the next. Building the ICW was mainly a matter of cutting through those necks and deepening the lagoons, rather than digging a canal across open country.
Albert Gallatin's Four Necks of Land, 1808
Treasury Secretary Albert Gallatin was the first to describe this geography as a national plan. In his 1808 report to the Senate he pointed out that a protected tidewater route from Massachusetts to Georgia was "principally, if not solely" interrupted by four necks of land. These were the Isthmus of Barnstable on Cape Cod, the strip of New Jersey between the Raritan and the Delaware, the Delmarva peninsula between the Delaware and the Chesapeake, and the low, marshy tract separating Chesapeake Bay from Albemarle Sound. Cutting these four, at an estimated cost of about three million dollars, would let a sea-going vessel travel inland from Boston harbour to the Carolinas and on through the island chain to St. Marys, Georgia.
Gallatin's full 20-million-dollar, ten-year transport programme was never approved. The British blockade in the War of 1812 showed how useful a sheltered coastal route would be, however, and each of his four cuts was eventually built. The Delaware and Raritan Canal was later abandoned, but the other three remain part of the waterway. After the General Survey Act of 1824, the Army Corps of Engineers took charge of federal navigation work. Congress set a national policy for an intracoastal channel from Boston to the Rio Grande in the Rivers and Harbors Act of 1909.
Cape Cod Bay to Manasquan Inlet: The Northern Open-Water Leg
North of New Jersey there are no continuous lagoons, so the waterway uses enclosed sounds and bays. From Manasquan Inlet the route crosses New York Bay, follows the East River into Long Island Sound, then passes Block Island Sound, Rhode Island Sound and Buzzards Bay. It cuts through the Isthmus of Barnstable by the 11.9 km Cape Cod Canal, crosses Cape Cod Bay and Massachusetts Bay, and ends with an 8 km inland passage up the Annisquam River between Gloucester and Annisquam. The Cape Cod Canal is about 146 m wide and 9.8 m deep, saves up to 217 km around the cape for about 14,000 users a year, and has a tidal current of up to 8.4 km/h.
Smaller boats can stay inside along New Jersey. The New Jersey Intracoastal Waterway runs 189 km from Manasquan Inlet through the Point Pleasant Canal and Barnegat Bay to Cape May Harbor and the Cape May Canal. It suits vessels drawing between about 1.8 m and 3.7 m, while larger commercial vessels generally go offshore.
Delmarva and the Chesapeake & Delaware Canal
Gallatin's third neck is the narrow strip of Delmarva between the head of Chesapeake Bay and the Delaware River. The mapmaker Augustine Herman had noted in the mid-17th century how close the two waters came, and Benjamin Franklin was among those who revived the idea in 1788. The Chesapeake & Delaware Canal finally opened in 1829 at a cost of $3.5 million. Today it is 22.5 km long, 137 m wide and 10.7 m deep, and it saves ships about 480 km between Philadelphia or Wilmington and Baltimore. South of it, the waterway follows Chesapeake Bay itself, the largest estuary in the United States at about 11,600 km², down to Hampton Roads.
Mile 0 at Norfolk: The Great Dismal Swamp and Albemarle Sound
The Atlantic ICW's official mileage starts at milepost 0.0 in Norfolk–Portsmouth, Virginia, at the fourth of Gallatin's necks. Between the Elizabeth River and Albemarle Sound lies a low peat swamp, and boats have two ways through it. The older is the 35 km Dismal Swamp Canal along the swamp's eastern edge. George Washington proposed it after visiting the Great Dismal Swamp in May 1763, the Dismal Swamp Canal Company was formed in 1784, and digging began in 1793. The canal was dug entirely by hand, mostly by enslaved labourers rented from nearby landowners, and opened in 1805. It is the oldest continually operating artificial canal in the United States.
The second route is the Albemarle and Chesapeake Canal, built by a private company between 1856 and 1860. It has two short cuts 48 km apart. One, 13.7 km long, joins the Elizabeth River to the North Landing River in Virginia, and the other, 8.9 km long, links Currituck Sound with the North River in North Carolina.
Pamlico Sound to Florida's Indian River Lagoon
South of the Virginia line the natural inside route is at its widest. Albemarle, Currituck, Croatan, Roanoke, Pamlico, Core and Bogue sounds together form the Albemarle-Pamlico system, with more than 7,800 km² of open water. That makes it the second-largest estuary in the United States after Chesapeake Bay. Pamlico Sound alone is about 130 km long and 24 to 32 km wide, the largest lagoon on the North American East Coast, and is closed off from the ocean by the Outer Banks. The ICW crosses these wide sounds before turning into narrower cuts and tidal creeks behind the islands of the Carolinas and Georgia.
In Florida the route follows another lagoon chain. The Indian River Lagoon is made up of Mosquito Lagoon, the Banana River and the Indian River. It is 0.8 to 8 km wide, averages only 1.2 m deep, and is home to more than 4,300 species of plants and animals. Further south the waterway passes Fort Lauderdale on its way to Key West, where the Atlantic section ends.
Florida's Missing Links: Okeechobee and the Cross Florida Barge Canal
Florida's geography also explains why the ICW is not one continuous channel. The waterway has three separate parts: Key West to Norfolk, Tarpon Springs to Fort Myers on Florida's Gulf coast, and Carrabelle in the Florida Panhandle to Brownsville. Two links were planned to join them. One was a dredged channel up Florida's Gulf coast from Tarpon Springs to St. Marks. The other was the Cross Florida Barge Canal across the north of the peninsula, an idea first put forward by Philip II of Spain in 1567. Work on the barge canal was funded and defunded from 1933 onward until it was cancelled in 1971, after opposition led by Marjorie Harris Carr to protect the Ocklawaha River valley and Florida's water supply. Congress formally deauthorised it in 1990, and the corridor is now a state greenway named after Carr.
Boats crossing the peninsula use the Okeechobee Waterway instead. Finished in 1937, it runs from Stuart on the Atlantic side through the St. Lucie Canal, across Lake Okeechobee and down the Caloosahatchee River to Fort Myers. The Corps of Engineers operates five locks and dams on the route, which takes tows up to 15 m wide drawing less than about 3 m.
Harvey Lock to Brownsville: The Gulf Intracoastal Waterway
West of Florida the waterway changes character. Along the Gulf Coast it is mainly a barge canal through the marshes and bayous of the Mississippi delta country and the lagoons of Texas. Distances are measured in statute miles east or west of Harvey Lock at New Orleans. For example, the Hathaway Bridge at Panama City, Florida, is at mile 284.6 EHL, and the Queen Isabella Causeway at South Padre Island is at mile 665.1 WHL. Congress authorised the Louisiana and Texas section in 1925. A 2.7-by-30.5-metre channel between New Orleans and Corpus Christi was finished in 1942, after German submarines had sunk numerous merchant ships off the East Coast, and the Gulf waterway was completed in 1949 with a controlling depth of 3.7 m.
At its western end the channel was dredged through the Laguna Madre, a shallow, hypersaline lagoon about 210 km long behind Padre Island. The Saltillo Flats, a land bridge about 32 km long, divide the lagoon into upper and lower halves, and the waterway joins the two. Along the route the waterway connects the ports of Houston, Corpus Christi, Beaumont, Lake Charles, Mobile and New Orleans, and reaches the Mississippi, Atchafalaya and Sabine rivers.
Federal law calls for a 3.7 m channel along most of the ICW, but funding has never matched it. Some stretches still have the 2.1 m or 2.7 m depths of earlier work, and larger vessels meet shoals and shallow water on several sections. The waterway is free to use, but since the Inland Waterways Revenue Act of 1978 commercial operators have paid a barge fuel tax. It began at 4 cents a gallon in 1980 and was raised in steps to 20 cents by 1995, with half of new construction paid from the trust fund it supports. In autumn and spring the channel also carries "snowbird" boaters moving south for the winter and north for the summer, and the Atlantic and Gulf sections form part of the 9,700 km Great Loop that circles the eastern United States through the Great Lakes and the Mississippi. If you want to know how the sand ridges that protect the whole route form and migrate, read our explainer on what a barrier island is.